Effective Isotropic Radiated Power (EIRP) is simply the amount of power radiated out of the antenna, into the air. EIRP takes into account the losses in Transmission line and connectors and the gain of the antenna, and presents a value of power out of the antenna, expressed in decibels.

EIRP = (power of transmitter (dBm)) – (losses in transmition line (dBm)) + (antenna gain(dBm))

In this example I have used dBm but it is just as common to see dBw, you will never see dB as it is never used in engineering.

Decibels is a measure of the reference between input and output. dBm uses a reference od 1mW and dBw uses a reference of 1W.

dBm = 10 log(power out / 1mW)

and

dBw = 10 log(power out / 1W)

A transmitter with a 50W output can be expressed as a 16dBw output

16.9897 = 10 * log(50/1)